After studying law at the Charles-Ferdinand University in Prague, he joined the Austro-Hungarian foreign service in 1895 and was dispatched to the embassy in Paris. In 1899, he was sent to The Hague, but only three years later he had to resign as a result of a lung infection and retired to his Bohemian estates.
Since 1903 a member of the Bohemian Lower House and since 1912 also of the Upper House, Czernin became a champion of conservatism, upholding the monarchical principle and opposing universal suffrage. He regarded the nobility as the main pillar of the empire. A close friendship with Archduke Francis Ferdinand in 1913 brought Czernin back into the diplomatic corps, this time as ambassador to Bucharest, where he served until August 1916. Czernin was a leading member of the heir apparent's so-called Belvedere Circle.
Emperor Charles appointed Czernin foreign minister on December 22, 1916. More energetic, more flexible, and more original than either Count Leopold von Berchtold or Burian von Rajecz, Czernin was at the same time more unpredictable, more volatile, and also more neurasthenic than his predecessors. Although General Erich Ludendorff managed to convince Czernin of the necessity to resume unrestricted submarine warfare in February 1917, the foreign minister spent much of that year attempting to persuade German military leaders as well as. the emperor of the necessity of peace even if this should mean Germany's loss of Alsace-Lorraine and restoration of Belgium's independence. Czernin hoped to wring these concessions from Berlin in return for large territorial gains in Poland. Conversely, Czernin sought to maintain inviolate the borders of Austria-Hungary, indeed, even to add Serbia and Rumania to a proposed Austrian Danubian Federation.
In April 1917, Czernin transmitted through Emperor Charles to Matthias Erzberger of Germany a gloomy prognostication outlining the reasons why the Dual Monarchy could not survive another winter of fighting. This document encouraged the German Parliament to pass its well-meaning but ineffective peace resolution in July 1917. Czernin climaxed his career by signing peace treaties with the Ukraine (February 9, 1918), Russia (March 3, 1918), and Rumania (April 14, 1918), although in each case bitter wrangling with the German ally preceded final accord. Czernin resigned his post on April 14, 1918, as a direct result of the notorious "Sixtus" affair. The foreign minister had been aware of Emperor Charles' secret negotiations with his brother-in-law, though not of the exact wording of the letter of March 24, 1917, in which the emperor had spoken of France's just demand for the return of Alsace-Lorraine. Revelation of this correspondence by Georges Clemenceau made Czernin's position untenable.
Ottokar Czernin retired in Austria after Czechoslovakian nationalist agrarian reforms deprived him of his lands in Bohemia. From 1920 to 1923 he served as deputy of the Democratic party in the Austrian National Council. He died in Vienna on April 4,1932.
During the revolution, the Czechoslovakian nationalist agrarian reforms deprived him of his lands in Bohemia and he withdrew to Salzkammergut in Austria. From 1920 to 1923, he served as a deputy of the Demokratische Partei in the National Council of the Republic of Austria.
In 1917, he was bestowed with the Grand Cross of the Order of Saint Stephen and invested as a Knight of the Order of the Golden Fleece. Apparently he wrote to Empress Zita after the war asking her not to expel him from the latter order because of his erratic behaviour as Foreign Minister.
In 1919, he published his memoirs of his days as an insider in the Austro-Hungarian political and diplomatic arenas during World War I, called In the World War, an interesting look at the inside machinations of an ancient empire being pulled apart by war.
Count von Czernin died in Vienna on 4 April 1932.
In 1897, he married Marie née Gräfin Kinsky von Wchinitz und Tettau (1878–1945) in Heřmanův Městec (German: Hermannstädtel).